1. Field of the Invention
The present invention relates to a solid-state imaging element and camera system for performing shuttering based on rolling shutter.
2. Description of the Related Art
Global shutter and rolling shutter (focal plane shutter) are known as types of electronic shutter for use in image sensors having photoelectric conversion elements arranged in a matrix form.
Global shutter is a type of electronic shutter that operates on all the pixels at the same time, whereas rolling shutter is a type of electronic shutter that operates on the pixels row by row.
In many cases, the pixels are read row by row with both global shutter and rolling shutter. With rolling shutter, the row on which an electronic rolling shutter operates and that which is read are shifted over time.
FIG. 1 is a diagram illustrating, for convenience, shuttering and reading in rolling shutter.
In FIG. 1, the horizontal axis represents the time, and the vertical axis the addresses of the read and shuttered rows. The unit for the horizontal axis is the horizontal read period (H) that is the read time for a single row.
In the example shown in FIG. 1, the storage time is denoted by Tint. The times at which the same address is accessed differ by time Tint between the read row and shuttered row.
For the shuttered row and read row, Ln rows are shifted from address LS to address LE every horizontal read period (1H), selecting one row after another.
In this case, the shuttering ends by time Tint earlier than the reading.
If the shuttering ends earlier than the reading as described above, the reading and shuttering are conducted at the same time in some rows, and only the reading is conducted in some other rows depending on the addresses.
In the example shown in FIG. 1, the shuttering and reading are conducted at the same time during period t1. However, only the reading is conducted during period t2.
Thus, it is known that long and narrow horizontal noise called a shutter level difference or FIBAR (Fixed Integration BAR) occurs if the number of times the shutter is operated changes while the reading is in progress.
This is attributable to the change in read output value caused by the difference in power load between when the reading and shuttering are conducted at the same time and when only the reading is conducted.
A solution to the above problem is disclosed in Japanese Patent Laid-Open No. 2001-8109 (hereinafter referred to as Patent Document 1) which maintains the power load constant by shuttering dummy rows.
This method is designed to shutter dummy rows after the shuttering of addresses LS to LE ends, thus maintaining the power load constant in periods t1 and t2 during which the reading is conducted.
The method described in Patent Document 1 is effective when the storage time is always constant between consecutive frames.
However, if the storage time is different between frames, the number of times the shutter is operated together with reading changes from one row to another, resulting in a shutter level difference.
In order to solve this problem, Japanese Patent Laid-Open No. 2005-269098 (hereinafter referred to as Patent Document 2) discloses another method. This method maintains constant the number of times the shutter is operated by providing as many dummy pixels as or more dummy pixels than the number of times the shutter is operated for two frames and thereby performing shuttering for two frames at all times.
FIG. 2 is a diagram illustrating an example of shuttering and reading when shuttering for two frames is conducted at all times.
In this case, shuttering for two frames is conducted at all times by providing dummy shutters DST1 and DST2.
CMOS (Complementary Metal Oxide Semiconductor) image sensors (CISs) are characterized in that they allow read addresses to be set with relatively less restraint than CCD (Charge Coupled Device) image sensors.
For example, in addition to a function to read all the pixels, those sensors widely used today incorporate other features, including an “addition” function adapted to read a plurality of pixel signals at the same time and a “skipping” function adapted to read pixels intermittently by skipping some rows or columns.
Image sensors are known to suffer from a phenomenon called “blooming.” Blooming is a change in signal level caused by signal charge overflow from a saturated photodiode (hereinafter PD) to an adjacent PD.
When a rolling shutter is used in particular, blooming occurs during “skipping” if the charge stored in the unread pixels is not discarded as appropriate, thus resulting in degraded image quality.
In contrast, a method has been proposed to suppress blooming (refer to Japanese Patent Laid-Open No. 2008-193618). This method activates a shutter to discard the charge from the unread pixels.
When a shutter is operated to prevent blooming during “skipping,” or when “addition” is performed, a plurality of rows are selected at the same time.
FIG. 3 is a diagram illustrating an example of read and shuttered row addresses in which two rows are “added together” and half the rows are “skipped.”
In FIG. 3, the unit for the horizontal axis is the horizontal read period (H) that is the read time for a single row.
In FIG. 3, the row addresses “n+9” and “n+11” are selected at the same time, added together and read at time t5.
The row addresses “n+17” and “n+19” are shuttered for the frame being read. The row addresses “n” and “n+2” are shuttered for the next frame. The row addresses “n+21,” “n+23,” “n+4” and “n+6” are shuttered for anti-blooming purpose.